1. Field of the Invention
The present invention relates to a simulation method and simulation apparatus for executing device design and accurate circuit simulation of an LDMOSFET.
2. Description of the Related Art
In a normal metal-oxide-semiconductor field-effect transistor (MOSFET), the allowable applied voltage is limited to several volts. In a laterally diffused MOSFET (LDMOSFET), the applied voltage reaches several hundred volts. To ensure such a high allowable applied voltage, i.e., breakdown voltage, the LDMOSFET has a lightly doped n−-region (high-resistance portion) called a drift region 13 between a drain contact 11 and a channel region 15, as shown in FIG. 6. The LDMOSFET can also set a different breakdown voltage by adjusting the length or impurity concentration in the drift region 13 or the length of an overlap region 14 between a gate electrode 12 and the drift region 13 or forming a local oxidation of silicon (LOCOS) in the drift region.
The drift region 13 serves as a resistance for a carrier that flows through a channel formed in a semiconductor substrate. Electric charges that exist in this portion are also non-negligible and induce an additional capacitance. Hence, an operation simulation model of an LDMOSFET device (to be simply referred to as an LDMOSFET device model hereinafter) requires a simulation model that accurately models the drift region and overlap region, in addition to a simulation model of a conventional MOSFET device.
To describe the characteristic of an LDMOSFET device, it is necessary to accurately describe not only an external voltage but also the voltage dependence of the internal potential of a channel end.
Conventionally, two LDMOSFET device models have been proposed. The first model includes “MM20” and “MM31” which have been developed by Philips of the Netherlands (M. B. Willemsen, R. van Langevelde, and D. B. M. Klaassen, “High-Voltage LDMOS Compact Modelling”, Proc. NSTI, vol. 3, p. 714, 2006). In “MM20”, an expression is defined not only for a current flowing through a channel but also for a current in a drift region. The voltage of the channel end is obtained by iterative calculation so that the two currents obtain the same value. The characteristic of an LDMOSFET device is described using the voltage. That is, separately taking the LDMOSFET device characteristic in the channel region and that in the drift region into consideration, the “MM20” and the “MM31” that describes a LOCOS portion are combined using a macro model for various breakdown voltages, thereby modeling an LDMOS device.
In the MOSFET device model, the characteristic of the LDMOSFET device is expressed by a combination of two transistors. This makes the calculation complex, and the characteristic cannot be determined uniquely.
When the drift region for raising the breakdown voltage of the LDMOSFET is considered as another transistor, the influence of the transistor performance of this portion becomes large. This departs from the device physics that an actual MOSFET is governed by carrier traveling in the channel portion. That is, it violates the principle that the MOSFET device characteristic is actually determined in the channel region.
The second model is “HV-EKV” developed by the Ecole Polytechique Federale de Lausanne, Switzerland (Y. S. Chauhan, F. Krummenacher, C. Anghel, R. Gillon, B. Bakeroot, M. Declercq, and A. M. Lonescu, “Analysis and Modeling of Lateral Non-Uniform Doping in High-Voltage MOSFETs”, Tech. Digest IEDM, p. 213, 2006). In the “HV-EKV”, a drift region for ensuring a high breakdown voltage is described as an external resistance using a macro model, and various characteristics except the resistance are added to this portion. However, because of simplicity and insufficient accuracy, the model cannot sufficiently describe the intrinsic device characteristic.
More specifically, the macro model of the high breakdown voltage portion includes resistances and capacitances and cannot describe the intrinsic complex voltage characteristic of the device. It is therefore impossible to reproduce the actually measured transient response.
“HiSIM” (Hiroshima-Univ. STARC IGFET Model) that is a bulk MOSFET model employs, however, a method of calculating a surface potential using a single expression for transistor (MOSFET) operations including depletion, weak inversion, and strong inversion and obtaining a current using a Poisson's equation (M. Miura-Mattausch, N. Sadachika, D. Navarro, G. Suzuki, Y. Takeda, M. Miyake, T. Warabino, Y. Mizukane, R. Inagaki, T. Ezaki, H. J. Mattausch, T. Ohguro, T. Iizuka, M. Taguchi, S. Kumashiro, and S. Miyamoto “HiSIM2: Advanced MOSFET Model Valid for RF Circuit Simulation”, IEEE Trans. Electron Devices, vol. 53, p. 1994, 2006). A MOSFET voltage-current characteristic obtained by this method can satisfactorily reproduce an actually measured value by relatively simple calculation. However, the above-described modeling of a drift region or modeling of an overlap region is not taken into consideration. For this reason, the model cannot accurately cope with an LDMOSFET and must be extended to an LDMOSFET model capable of covering the structure of an LDMOSFET.